Extend Metasql query builder section on DATA page

[KLarpen]

Current version of Metasql query builder section contains mostly initial setup guide. There is only one usage example without any description. I agree that select example is self explanatory, but it would be better to say more about possibilities of Metasql at least as a teaser.

This PR adds overview of the Metasql.Database methods and clearly separates initial setup guide. That changes I had organized in one commit. However I plan to add some more explanatory to Metasql template sql as a separate second commit in this same pull request. That way it might be consistent update with open possibility to not land second commit.

[KLarpen]

The second commit with enhancement of Metasql template sql section has been pushed.

[KLarpen]

@tshemsedinov please review the Doc's extention proposal.